1. Field of the Invention
The present invention relates to a power control system for a vehicle ac generator that controls output power at a high rotation speed.
2. Description of the Related Art
Recently, output power of a vehicle ac generator has increased and engine idling speed, which is the lowest generating speed of the ac generator, has been lowered. Increase in the output power of the ac generator at the lowest speed necessarily results in excessive output power at the highest speed of operation of the ac generator. This, in turn, increases amplitude of surge current and necessitates increasing the capacity of wire harnesses and an electric fuse of the vehicle.
JP-A 57-106400 proposes a field-current-duty-cycle control system for limiting the output power of an ac generator at a high rotation speed. In such a system, when the rotation speed becomes higher than a predetermined threshold level, width of a pulse signal is fixed and cycle time thereof changes in inverse proportion to the rotation speed to turn off a switching element of a regulator. Therefore, the turn-off period is fixed and the turn-on period (or duty cycle) is equal to the difference between the cycle time and the turn-off period.
Because the duty cycle is set as soon as the rotation speed becomes higher than the threshold level, the duty cycle is reduced abruptly. This reduces the output power of the generator abruptly, thereby giving shock to the generator.
JP-B2-4-47548 proposes a field-current control system, in which the field current is controlled so that the amplitude of surge current becomes lower than a breakdown voltage of a switching element even if the rotation speed of an ac generator becomes higher than a predetermined value. JP-A-3-49599 proposes a control system for limiting the maximum duty cycle. The control system has a specific field current source that supplies the field coil with current at a voltage higher than the output voltage of the generator and the switching element of a regulator with pulse signals so that the maximum duty cycle of the switching element can be controlled according to the rotation speed of the generator. In these two systems, triangular or saw tooth voltage signals are generated in proportion to the rotation speed, and the predetermined value is detected when the triangular voltage signal becomes higher than the predetermined voltage.
However, it is very difficult to provide an optimum characteristic curve for the maximum duty cycle to change in response to the rotation speed. In particular, if the predetermined voltage is higher than the peak voltage of the triangular voltage signal, the maximum duty cycle becomes zero. If the predetermined value is set lower than the bottom of the triangular voltage signal, the maximum duty cycle becomes 100%. This cause abrupt change of the duty cycle and gives shock to the generator.